This invention relates in general to magnetic devices, such as magnetic recording disks and heads and magnetic tunnel junctions, and more particularly to magnetic devices that use exchange-coupled antiferromagnetic/ferromagnetic (AF/F) bilayers.
The exchange biasing of a ferromagnetic (F) film by an adjacent antiferromagnetic (AF) film is a phenomenon that has proven to have many useful technological applications in magnetic devices, and was first reported by W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1959). Whereas the magnetic hysteresis loop of a ferromagnetic single layer film is centered about zero field, a F/AF bilayer often will show an asymmetric magnetic hysteresis loop which is shifted from zero magnetic field in the plane of the film by an exchange-bias field, HB. The direction of the exchange bias field within the plane of the film can be varied. In particular, this direction can be set during the growth of the AF film and is determined by the orientation of the magnetic moment of the F film when the AF film is deposited on top of the F film. The direction of the exchange bias field can also be changed by heating the F/AF bilayer after growth above the so-called blocking temperature, TB, of the AF film. The blocking temperature is typically close to but below the Nxc3xa9el or magnetic ordering temperature of the AF film. In addition to an offset of the magnetic hysteresis loop of the F film, the F film in a F/AF bilayer typically shows an increased coercivity below TB. The detailed mechanism which determines the magnitude of the exchange bias field and the increased coercive field is still a matter of considerable debate but it is generally agreed that these effects arise from an interfacial interaction between the F and AF films. See, for example the articles by J. Noguxc3xa9s and I. K. Schuller, J. Magn. Magn. Mat. 192, 203 (1999); and A.and K. Takano, J. Magn. Magn. Mat. 200, 552 (1999). Exchange-biased thin films have found several important applications, especially for magnetic recording read heads. In particular, exchange biasing can be used to locally harden or stiffen the magnetic response of, for example, the edges of a ferromagnetic sensing film in an anisotropic magnetoresistive read head. Exchange-biased ferromagnetic films can also be used to provide local magnetic fields such as those required for optimally biasing magnetoresistive read sensors. Exchange biasing is also an integral component of giant magnetoresistive spin-valve heads. More recently, exchange biasing has been used to engineer magnetic tunnel junction devices for use in magnetic recording read sensors applications and in magnetic random access memories, as described for example in IBM""s U.S. Pat. No. 5,650,958.
Although exchange biasing has been widely studied and has found important technological applications, the phenomenon of exchange biasing has only previously been observed in F/AF thin film systems in which the moment of the ferromagnetic film lies in the plane of the film. For many applications, particularly for advanced magnetic recording media and advanced magnetic recording read heads, it would be extremely useful to be able to exchange bias ferromagnetic films whose moments lie perpendicular to the plane of the film.
What is needed is a means of providing perpendicular exchange bias for perpendicularly magnetized ferromagnetic films.
The invention is a magnetic device that includes a ferromagnetic/antiferromagnetic (F/AF) structure wherein the ferromagnetic layer has perpendicular magnetic anisotropy by being exchange coupled with the antiferromagnetic layer. When used in perpendicular magnetic recording disks, the F/AF structure can be a Co layer and a CoO AF layer located on top of the Co layer where the Co layer is the top layer in a series of Co/Pt bilayers, or a NiO AF layer and a Co layer located on top of the NiO layer, where the Co layer is the bottom layer in a series of Co/Pt bilayers. When used in a magnetic tunnel junction read head, the F/AF structure is the ferromagnetic free layer that is longitudinally biased at its edges by being perpendicularly exchange coupled to AF insulating oxide layers at these edges. When used in magnetic tunnel junction memory cells the F/AF structure is the fixed magnetic layer and a AF conducting layer where the AF layer exhibits perpendicular exchange biasing to the adjacent fixed ferromagnetic layer.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.